Hi Mark,
we usually release a "major" version of the app when Moodle releases a new major version. So when Moodle 3.6 is out we'll release Moodle Mobile 3.6 (we usually release it 1 or 2 weeks later, depending on the amount of work we have).
Regarding minors, we also try to match Moodle minors too, although sometimes we release more if there's any major bug. Like above, sometimes there's a bit of delay between the Moodle minor and the Mobile minor releases.
This policy was broken during the Ionic 3 migration, we skipped a minor version in 3.4 and there was a huge delay after Moodle 3.5 release. AFAIK we'll resume the schedule above after 3.6.
Cheers,
Dani